1. Field of the Invention
The present invention relates to piezoelectric actuators and more particularly to a capacitive sensor system for more accurately moving such actuators to a desired position.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The use of piezoelectric electric actuators to achieve small controlled movement is not new. For example, U.S. Pat. No. 5,511,931 to T. Arai, et al. discloses the use of six piezoelectric electric actuators for moving a micromotion stage. A holder fixed at one end of the stacked piezoelectric electric element supports a capacitance type displacement gage and a target member so as to form a gap between the two. A change in the distance between the holder and target member caused by expansion or contraction of the piezoelectric electric element is indicated by the capacitance type displacement gage.
Another use of piezoelectric electric devices for small movement is disclosed in U.S. Pat. No. 5,075,600 to El-Hamamsy, et al. According to this patent, an electrodeless high-intensity discharge lamp ballast uses a variable capacitor in series with the excitation coil of the lamp. The variable capacitor includes a fixed conductive plate and a movable conductive plate with a dielectric material, such as Teflon, synthetic resin polymer, mica, or Kapton polyimide film disposed between the two plates. The piezoelectric electric actuator is used to vary the distance between the two conductive plates which changes the capacitance in inverse proportion to the distance moved. Thus, the movable plate is moved from a starting position to a running position to maintain the ballast load in "tune" under both the starting and running conditions.
As was discussed above, piezoelectric electric devices can be used as small actuators to produce very small movements of small components. The amount of movement or final position of the actuator is directly controlled by the level of voltage applied. Unfortunately, movement of the piezoelectric electric actuator is not linear with respect to the applied voltage.
Therefore, to accurately position piezoelectric electric actuators in the past, a position translator such as an encoder was attached to the actuator, and the output of the encoder was then used in a feedback link to indicate when the correct position was achieved. Unfortunately, such encoder systems are not only expensive, but are also extremely impractical for use with an array of such piezoelectric electric actuators which are packed close together. Other attempts to provide the necessary feedback have included the use of a potentiometer which acts as a position indicator. Although less expensive than an encoder, the use of a potentiometer is still impractical to use with a large array of such small actuators, and does not work well with the small movements created by a piezoelectric electric actuator.
There are also examples of capacitance sensor systems available in the prior art. For example, see U.S. Pat. No. 5,594,353 issued to B. D. Hemphill which measures the change in capacitance values in a pair of capacitors which share a common electrode.
U.S. Pat. No. 5,633,594 issued to H. Okada describes a capacitor sensor system which applies first and second driving signals which are opposite in face to a sensor capacitor and a reference capacitor, respectively. The two capacitors are in turn connected to circuitry which generates a signal in response to the differences in potential.
U.S. Pat. No. 5,786,698 to R. E. Mallory discloses apparatus for boot strapping an amplifier of a capacitive displacement transducer. The apparatus replaces a transformer with two capacitors and two resisters configured to provide biased supply voltage. This arrangement provides similar performance at a reduced size and cost.